The author of Stinking Rich 3/Tory party chairman, who attends Cabinet as minister without portfolio, last month denied that he had continued to work as an internet marketer after being elected in 2005, telling LBC radio: "To be absolutely clear I don't have a second job and I have never had a second job whilst being an MP. End of story."

But a recording obtained by The Guardian captures the MP in 2006 selling a business self-help guide and claiming his products could make listeners a "ton of cash by Christmas".

The Green/Shapps collective initially dismissed the issue as an "old story", but the Conservatives said that although the chairman had talked of his writing career having ended when he became an MP "in fact it ended shortly afterwards". The BBC later reported that the Shapps Borg told its correspondent he had "screwed up" on dates and stated his case "over-firmly" in the LBC interview.

A Conservative party spokesman said: "Like many authors and journalists, Grant wrote with a pen name. This was completely transparent: his full name and biographical details were permanently published on the company's main website. Given that this was a decade ago, and was mentioned during the cut and thrust of an interview, he referenced that his writing career had ended when he became an MP, in fact it ended shortly afterwards."

In an online interview with the BuzzFeed website, Cameron said: "Grant did have another job when he first became an MP and he declared that in the Register of Members' Interests which is what you are meant to do. But he obviously made a mistake by saying in some interviews that the work had stopped earlier than it had."